Adjust buffer size to avoid truncation.

tree/treeplayer/src/TTreeFormula.cxx:2550:23: warning: ‘char* strncat(char*, const char*, size_t)’ output may be truncated copying between 0 and 1023 bytes from a string of length 1023 [-Wstringop-truncation]
                strncat(second,work,kMaxLen-1-strlen(second));
                ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~